Highly efficient nonlinear vortex beam generation by using a compact nonlinear fork grating.

Vortex beams with an orbital angular momentum (OAM) are extremely important in optical trapping, optical micromachining, high-capacity optical communications, and quantum optics. Nonlinear generation of such a vortex beam enables vortex beams to be obtained at new wavelengths, which opens up new possibilities for all-optical switching and manipulation of vortex beams. However, previous nonlinear vortex beam generation suffers from either low efficiency or low-level integration. Here, we use the technique of ultraviolet photolithography-assisted inductively coupled plasma (ICP) etching to realize a compact nonlinear fork grating for high-efficiency nonlinear vortex beam generation. In our experiment, the depth of such a compact nonlinear fork-grating structure can be precisely controlled by etching time. The vortex beams with a topological charge of l = ±1,  ± 2,  ± 3 can be generated in the far field, and the normalized nonlinear conversion efficiency of such nonlinear vortex beam is 189% Wcm. Our method not only provides an efficient and compact method for nonlinear vortex beam manipulation but also suits for timesaving and large-area nonlinear functional device fabrication.